The present invention relates to a new and improved vehicle power steering system and, more specifically, to a vehicle power steering system which includes a power steering pump, a power steering motor for turning steerable vehicle wheels, and a power steering valve for controlling fluid pressure supplied by the power steering pump to the power steering motor.
The general construction and mode of operation of a vehicle power steering system is well known. Upon rotation of a vehicle steering wheel, a pinion meshes with the teeth of a rack bar, thus moving the rack axially. Also, a power steering valve directs fluid pressure supplied by a power steering pump to one of two motor cylinder chambers of a power steering motor. The other motor cylinder chamber is connected with a reservoir through the power steering valve. This results in the hydraulic assistance of the power steering motor to turn steerable vehicle wheels.
The motor cylinder chambers have seals which are urged into sealing engagement with components of the power steering motor by the fluid pressure in the motor cylinder chambers. When the fluid pressure in the motor cylinder chambers falls below a predetermined pressure such as 40 psi, the fluid pressure in the motor cylinder chambers is not great enough to urge damaged or otherwise imperfect motor cylinder seals into sealing engagement with the components of the power steering motor. Thus, fluid may leak from the power steering motor and/or foreign material may enter the motor and further damage the seals. When the power steering pump is not driven, such as when the engine is turned off, the fluid pressure within the motor cylinder chambers typically falls below the predetermined pressure. The fluid pressure within the motor cylinder chambers also may fall below the predetermined pressure when the power steering valve which may be "open center" is in a neutral position directing fluid pressure from the power steering pump to both motor cylinder chambers equally.